Organosiloxane polymers are prepared on an industrial scale using two basic processes. The most widely practiced process is known as the equilibration process which involves the rearrangement of siloxane bonds to form an equilibrated mixture. The term equilibration is used to describe the phenomenon which exists when the ratio of linear organosiloxane polymers to cyclic organosiloxane oligomers is maintained at a constant value. When dimethyl siloxane is polymerized, equilibrium is reached when the ratio of linear polymer to cyclic oligomers is about 86:14 on a weight-weight basis. A constant ratio of linear to cyclic organosiloxanes is always reached even when the starting material is a cyclic organosiloxane, a mixture of cyclic material and linear material or an all linear monomer or oligomer.
Equilibration is reached by the use of a wide variety of acidic or basic materials as catalysts. During the equilibration process, a constant breaking and forming of siloxane bonds takes place until the equilibrium point is reached. The massive breaking and forming of siloxane bonds permits the use of chainstoppers which will react to form a terminal non-reactive group on the end of the polysiloxane molecule. The cyclic oligmers are removed from the reaction mixture at the end of the equilibration process by a stripping process after deactivation or removal of the catalyst. The formation of cyclics is a substantial drawback because it adds to the cost of the process and extends the time necessary to complete the processes.
An alternative process for producing organosiloxane polymers is condensation which promotes the head to tail condensation of siloxane monomers or oligomers. The condensation processes generally depend on the removal of water to form higher molecular weight polymers. Cyclics are not produced in the condensation process. Useful condensation catalysts include phosphorus nitrogen compounds (PNC). Relatively mild acids and bases have also been used as condensation catalysts and strong acids and bases have been used as condensation catalysts at temperatures that will not result in equilibration and the formation of cyclic organosiloxanes. The general procedure which is used in the condensation process is to combine one disilanol monomer or oligomer with an acidic, basic or PNC catalyst and after the desired polymer has reached the desired molecular weight, the polymerization is terminated. The reaction may be terminated by deactivating the catalyst using a suitable alkaline or acidic material. The neutralization will prevent further polymerization and will permit the use of the polymer without further purification.
In the prior art patents which describe condensation reactions with PNC catalysts, there was a recognition that in addition to the condensation reaction that was taking place, rearrangement of the siloxane could take place with little or no formation of cyclic siloxanes. In other prior art, there was little or no recognition that certain other known condensation catalysts of the family of acids, such as sulfuric acid, fluorinated alkanesulfonic acids, or clays, especially kaolin, function without substantial increase in cyclic contents. With respect to the PNC catalysts, in U.S. Pat. No. 2,830,967 there is described a process which is based on the use of a PNC catalyst and a liquid organosiloxane. This process is described as causing the organosiloxane polymers to "condense and rearrange" and it is disclosed that the catalyst may be left in the final product because it has no "depolymerizing effect". The examples and text of U.S. Pat. No. 2,830,967 all disclose the use of a single siloxane which is polymerized to form a product having a higher molecular weight. GB 2 252 975A discloses a method of making organosiloxanes using a complexed PNC catalyst of the formula [X(PNX.sub.2 =N).sub.n PX.sub.3 ]+B where X is a halide atom and B is a complexing agent comprising an anion of a Lewis acid. This patent states that traces of catalyst may re-equilibrate an organopolysiloxane to produce cyclics and for this reason the traces of the catalyst should be neutralized.
U.S. Pat. No. 5,008,229 discloses a process for the condensation of one organosiloxane having silicon bonded hydroxyl groups or a mixture of organosiloxanes having silicon bonded hydroxyl groups in the presence of a PNC catalyst and a solubilizer. The text of U.S. Pat. No. 5,008,229 points out that the condensation process is always carried out under reduced pressure, and this will be expected to lead to high molecular weight products, such as gums instead of disproportionated products.
U.S. Pat. No. 5,009,934 discloses the reaction of a high molecular weight dihydroxydimethylpolysiloxane and 1,3-dihydrogentetramethyldisiloxane in the presence of PNC to produce a complex mixture of hydride-terminated polysiloxanes. This procedure requires that at least one of the organosiloxanes must be hydrogen terminated and the product is also hydrogen terminated.
The applicants have discovered that if two or more different organosiloxanes, which have different molecular weights, are combined at weight ratios of from about 1:99 to 99:1 or more preferably from 5:95 to 95:5 in the presence of a judiciously selected condensation/disproportionation catalyst, as will be described later, e.g., acids, some types of kaolin, a PNC-, or oligomerized PNC catalyst, a complexed PNC catalyst, an acid derivative of a PNC catalyst, mixtures of any of them, and the like, a reaction will take place which will result in the formation of a different organosiloxane which has a molecular weight lower than the molecular weight of the organosiloxane reactant which has the highest molecular weight of one of the two starting organosiloxanes. In a two-step modification of the process, one of the reactants is first made by condensing a high silanol content polydiorganosiloxane, followed by disproportionation of the mixture, as described. In any event, the reaction does not result in the formation of any significant quantities of cyclic siloxanes, i.e. less than 6 wt %, preferably below 2 wt %, and the products may be directly utilized for the formulation of commercial products without any substantial purification provided that any residual catalyst is neutralized.
The novel process of the invention is being characterized as a disproportionation reaction to point out that the product is a lower molecular weight product than one of the two starting materials. It is believed that the process of the invention depends upon the breaking and reforming of siloxane bonds but the applicants do not wish to be bound to any theory which may explain the results provided by the process of the invention.
It is therefore a primary object of the invention to provide a process for the preparation of organosiloxanes which does not result in the formation of any significant quantity of cyclic siloxanes.
It is also an object of the present invention to provide a process for the preparation of low viscosity organopolysiloxane fluids without the formation of any significant quantity of cyclic organopolysiloxanes.
It is also an object of the present invention to provide a process for the rapid preparation of a low viscosity polyorganosiloxane fluid with a normal molecular weight distribution by the disproportionation of two or more reactants having different molecular weights.
It is also an object of the present invention to provide a process for the preparation of a organosiloxane copolymer having silicon hydride group and silicon vinyl groups.
It is a further object to start with a low molecular weight, high silanol content siloxane material and produce low molecular weight triorganosiloxy terminated oils.
These and other objects of the invention will become apparent from the present specification.